There has been considerable effort devoted in recent years to solve various ecological and environmental problems such as air pollution, acid rain, etc. Combustion effluents and waste products from various sources are a major source of air pollution when discharged into the atmosphere. Unless the waste products are treated to remove deleterious components, the degradation of the environment will continue. Acid rain, forest and vegetation decline, changes in the ozone layer, harmful and irritating smog, etc., are examples of the results of the pollution of the atmosphere.
The common sources of pollution include internal combustion engines, industrial plants, utility boilers, gas turbines, and commercial establishments such as service stations, dry cleaners, etc. It has been estimated that power plants are responsible for about 1/3 of the annular NO.sub.x emissions while mobile sources such as automobiles and trucks are responsible for about 40% to about 50%. The types of air pollutants generated by such facilitates include particulate emissions such as coal ash, sulphur compounds such as SO.sub.2 and SO.sub.3, carbon monoxide, ozone, and nitrogen oxides, commonly referred to collectively as "NO.sub.x ".
One of the common components found in polluted air is nitrogen dioxide (NO.sub.2) which is known to be an extremely poisonous material. Nitrogen dioxide in introduced into the atmosphere from the various sources such as commercial plants products nitric acid, but a major source of nitrogen dioxide is from nitric oxide (NO) formed by combustion processes of the types described above. The nitrogen oxide is formed during such combustion processes by (1) the reaction of nitrogen with atmospheric oxygen in the high temperature portion of the flame ("thermal fixation"); and (2) the oxidation of organic nitrogen compounds in the fuel on burning. The nitric oxide formed on combustion is converted to nitrogen dioxide on contact with air in the atmosphere.
Various procedures have been suggested to remove the oxides of nitrogen from waste gases so that the gases may be discharged into the atmosphere without harm to the environment. Nitrogen oxides emissions from boilers, gas turbines and internal combustion engines have been reduced by modifying the design of the engine or boiler to be more efficient or to operate at a lower temperature. Other proposals for reducing nitrogen oxide emissions involve use of various chemicals to reduce the nitrogen oxide content of effluent gas by converting the nitrogen oxides to innocuous gases. Such chemical processes, however, generally require extremely high temperatures such as in the range of about 1600.degree. to about 2000.degree. F. and higher. The temperatures of some of these chemical reactions for reducing nitrogen oxide content have been reduced by utilizing catalysts which are effective in promoting the reduction of nitrogen oxide. Various techniques for reducing NO.sub.x emissions from various combustion processes are described in the article entitled "Reducing NO.sub.x Emissions," Power Sep. 1988, pp S-1 to S-13.
Among the chemicals which have been suggested as being useful in reducing the nitrogen oxide content of combustion effluents are nitrogen-containing compounds such as ammonia, urea, cyanuric acid, etc. For example, U.S. Pat. Nos. 3,900,554; 4,335,084; 4,743,436; 4,849,192; and 4,851,201 describe processes utilizing ammonia to reduce nitrogen oxide emissions. The use of urea is described in U.S. Pat. Nos. 4,208,386; 4,325,924; 5,719,092; and 4,851,201. The use of cyanuric acid, and more specifically, the decomposition product of cyanuric acid, isocyanic acid, for reducing the nitrogen oxide content of combustion effluents is described in U.S. Pat. Nos. 4,731,231; 4,800,068; and 4,861,567; and by R. A. Perry and D. L. Siebers, Nature Vol. 324, 18/25, pp 657, 658.
Various catalysts have been suggested as being useful for treating combustion gases and removing harmful components such as nitrogen oxide, carbon monoxide, sulfur trioxide, etc. The catalysts may be used alone or in combination with various chemicals such as ammonia. U.S. Pat. Nos. 4,874,590 and 4,867,954 describe the use of microporous molecular sieves with ammonia to remove nitrogen oxides and sulfur oxides from gas streams. Catalytic procedures for simultaneously removing nitrogen oxides, sulfur oxides and particulates from flue gases are the subject of U.S. Pat. Nos. 4,609,537; 4,617,175; and 4,692,318. The process described in the '537 patent initially involves removing nitrogen oxide with a nitrogen oxide-capturing reducing agent which may be hydrogen, ammonia, ammonia-liberating compounds, carbon monoxide, light hydrocarbon gases or stream. The gases are then treated with a sulfur oxide-capturing and particulate-removing material selected from the group of absorbers and absorbers with at least one promoter thereon. The absorbers comprise oxides of at least one metal selected from the group consisting of aluminum, bismuth, manganese, yttrium, antimony, copper, tin, rare earth metals, Groups Ia metals and Groups IIa metals. The promoter comprises at least one member, in a free or combined form, selected from the group consisting of rare earth metals, Group VIII noble metals, chromium, vanadium, rhenium, antimony, silver, and combinations thereof. Dysprosium is listed as one of the rare earth metals (Col. 12, line 41). The '175 patent describes and claims a system for removing nitrogen oxide, sulfur oxide and particulates utilizing the process described and claimed in the '537 patent. U.S. Pat. No. 4,692,318 describes a modification of the process claimed in the '537 patent utilizing similar catalysts.
U.S. Pat. No. 4,124,689 describes a ceramic mixed oxide catalyst containing rare-earth elements. In particular, the ceramic catalysts comprise a rare earth metal and a metal of the first transition series. Optionally, and preferably, the catalysts also contain zirconium, tin or thorium, or mixtures thereof and/or an alkaline earth metal. The catalysts can be employed in the catalytic removal of carbon monoxide, hydrocarbons, nitric oxides and sulfur dioxide from exhaust gases of generating or heating plants and internal combustion engines which burn fossil fuels. In Col. 10, beginning at line 27, various reactions are indicated as being catalyzed with the claimed catalysts, and one of the reactions is the reaction of carbon monoxide with nitric oxide.
U.S. Pat. No. 4,760,044 describes a process and catalyst for minimizing the emission of hydrogen sulfide from automotive exhausts. The catalytic composite which is used in the process comprises a mixture of (1) a primary refractory inorganic oxide support having dispersed thereon at least one rare earth oxide, and (2) a secondary support selected from the group consisting of zirconia, titania, ceria, silica, magnesia, natural and synthetic zerolites having dispersed thereon at least one second active component which is an oxide of a metal, the metal being selected from the group consisting of metals which form a stable sulfide under fuel-rich conditions, said mixture having at least one catalytic metal selected from the group consisting of platinum, palladium, rhodium, ruthenium and iridium deposited thereon. Examples of the second active metal component include nickel, zinc, cobalt, copper, and lead.
U.S. Pat. No. 4,791,091 describes catalyst for treating exhaust gases from internal combustion engines. The catalyst composite described in this patent comprises a support which is a refractory inorganic oxide having dispersed thereon lanthanum oxide, at least one other rare earth oxide and at least one noble metal component. Examples of the support include alumina, zirconia, silica, titania, etc. and examples of the noble metals include platinum, palladium, rhodium, ruthenium and iridium. Examples of the rare earth metal oxides include cerium, praseodymium, neodymium, dysprosium, europium, holmium and ytterbium.
Layered automotive catalytic composites are described in U.S. Pat. No. 4,868,148. The composites comprise a first support which is a refractory inorganic oxide having dispersed thereon at least one noble metal component and having dispersed immediately thereon an overlayer comprising at least one oxygen storage component and optionally a second support which is a refractory inorganic oxide. The oxygen storage component is an oxide of a metal selected from the group consisting of iron, nickel, cobalt and the rare earths with the rare earths being preferred. Specifically preferred rare earths are cerium, lanthanum, and mixtures of cerium and lanthanum.
U.S. Pat. No. 4,528,279 describes monolithic catalysts useful for purifying exhaust gases from internal combustion engines. The catalyst comprises an inert carrier; a film of alumina and rare earth oxides coated onto the surface of said inert carrier wherein cerium and lanthanum are present in said film in an atomic ratio of 0.05 to 0.3; and the film carries a catalyst component containing at least one element of the platinum family.
U.S. Pat. No. 4,919,902 also describes the catalyst for treatment of exhaust gases from internal combustion engines. The catalyst comprises a support of a refractory inorganic oxide coated with a combination of: lanthanum; at least one noble metal; and at least one other rare earth metal. U.S. Pat. No. 4,923,842 also describes a lanthanum-containing catalyst for removing nitrogen oxides from gases. The catalyst comprises a platinum and rhodium combination with a lanthanum oxide overlayer and an oxygen storage component which may be a rare earth metal oxide such as dysprosium acetate.